26 MEDICAL ENTOMOLOGY 



the pumping apparatus, by means of which the blood is drawn up from 

 the wound. It has already been stated that this is devel- 



The Sucking O ped from a part of the alimentary tract within the 

 Apparatus 



head. To understand the true nature of the pumping 



organ it is necessary to refer for a moment to the embryology of the parts. 

 The alimentary tract of insects is developed in three divisions, only 

 the middle one of which is lined with true digestive epithelium. The 

 anterior and posterior regions are formed by invaginations of the cuticle 

 and are therefore lined with chitin which is continuous with that of the 

 exo-skeleton. In most parts this chitin forms only a thin layer, but in 

 a part of the canal within the head the walls are strongly chitinized to 

 form the sucking chamber. It may be supposed that in the earliest forms 

 the whole of the tube was dilated to form a single chamber, and united 

 with the wall of the head capsule by means of radiating muscle fibres, 

 which, by drawing the walls apart, could produce a negative pressure 

 sufficient to draw up the fluid. In the course of the differentiation of 

 species the single cavity became divided into two, of equal or unequal 

 size, and a sphincter muscle, in some cases, developed between them. 

 (Plate III, fig. 5.) 



In all the Diptera these two chambers, in one form or another, can be 

 distinguished, and we may adopt for them the terms buccal cavity and 

 pharynx respectively. The original circular lumen is modified, in the 

 part which functions as a pump, by the moulding of the wall into two, or 

 in some cases three, strong chitinous plates, normally in contact with 

 one another but capable of being pulled apart by the dilator muscles. 

 In the Orthorrapha there are two functional pumps, while in the 

 Cyclorrapha the first is modified to form a connecting tube, without 

 dilator muscles. 



In Tabantis the two pumps are well developed. (Plate V, fig. 1.) The 

 buccal cavity is an oblong chamber composed of two flattened plates of 

 chitin, continuous, as previously stated, with the epipharynx and hypo- 

 pharynx of the food channel. At the posterior or upper end the walls 

 are produced into two stout cornua, between which the cavity opens into 

 the pharynx by a short and narrow channel surrounded by a sphincter 

 muscle. The buccal cavity, being in line with the food channel, is 

 perpendicular to the long axis of the body, while the pharynx is in line 

 with it, the channel between them opening into its middle portion. 

 It is formed by a single quadrilateral plate of chitin, the lateral angles of 

 which are turned downwards and connected with one another by a 

 membrane continuous with that of the channel leading from the buccal 



